The catalyst generally comprises a support (for example, formed from at least one refractory oxide, the support possibly also including one or more zeolites), at least one noble metal (preferably platinum), and preferably at least one promoter metal (for example tin or rhenium), at least one halogen and optionally one or more additional elements (such as alkalis, alkaline-earths, lanthanides, silicon, group IVB elements, non noble metals, group IIIA elements, etc.). Catalysts of this type contain platinum, for example, and at least one other metal deposited on a chlorinated alumina support. In general, such catalysts are used to convert naphthenic or paraffinic hydrocarbons, which can be transformed by dehydrocyclisation and/or dehydrogenation, for reforming or for the production of aromatic compounds (for example for the production of benzene, toluene, ortho- meta- or para-xylenes). Such hydrocarbons originate from fractionation of crude oil by distillation or other transformation processes.
Such catalysts have been widely described in the literature.
One way of increasing the yields of such reforming or aromatic compound production processes is to reduce the operating pressures at which the different reactions of interest are carried out. As an example, reforming reactions were carried out at 40 bars 30 years ago; 20 years ago, at 15 bars. Today, reforming reactors usually operate at pressures of less than 10 bars, in particular in the range 3 to 8 bars.
The improvement in desirable reactions due to a reduction in pressure is accompanied by more rapid deactivation of the catalyst by coking. Coke, a high molecular weight compound constituted essentially by carbon and hydrogen, is deposited on the active sites of the catalyst. The H/C molar ratio of the coke formed varies from about 0.3 to 1.0. The carbon and hydrogen atoms form condensed polyaromatic structures with a variable degree of crystalline organisation, depending on the function and nature of the catalyst and the operating conditions of the reactors. While the selectivity of transformation of the hydrocarbons to coke is very low, the amounts of coke accumulated on the catalyst can be large. Typically, for fixed bed units, such amounts are in the range 2.0% to 20.0% to 25.5% by weight. For circulating bed units, such amounts are below 10.0% by weight.
Coke deposition, which is more rapid at low pressure, also requires more rapid regeneration of the catalyst. Current regeneration cycles have become as short as 2-3 days.
Our European patent EP-A-0 378 482 discloses a continuous process for regenerating a reforming or aromatic compound production catalyst which can overcome the inherent disadvantages of shorter and shorter cycles. One of the regeneration steps is oxychlorination of the catalyst. The present invention concerns this step.
In EP-A-0 378 482, the used catalyst slowly travels from top to bottom in a regeneration vessel where it meets, in succession, a first radial moving bed combustion zone, a second radial moving bed combustion zone, an axial moving bed oxychlorination zone and an axial moving bed calcining zone, and:
a) in the first combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reforming reactor, at a temperature in the range 350.degree. C. to 450.degree. C., using a combustion gas based on an inert gas circulating as a co-current to the catalyst, comprising 0.01% to 1% of oxygen by volume, the combustion gas originating from a zone for washing the gases from the combustion, oxychlorination and calcining steps; PA1 b) in a second combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reactor, at a temperature which is higher by at least 20.degree. C. than the temperature in the first combustion zone, in the presence of gases originating from the first combustion zone and in the presence of an inert makeup gas to which up to 20% by volume of oxygen is added so that the catalyst is in contact with a gas comprising 0.01% to 1% by volume of oxygen, the gases circulating as a co-current with the catalyst; PA1 c) the burn gases are evacuated from the second combustion zone and sent to a washing circuit after first being mixed with the gases extracted from the oxychlorination zone and the calcining zone; PA1 d) in the axial oxychlorination zone, the catalyst is treated with a co-current of a mixture of a gas originating from the calcining zone and the chlorinated gas for 30 min to 60 min, the mixture forming an oxychlorination gas comprising 4% to 10% by volume of oxygen, at a pressure of 3 to 8 bars; the water content is of the order of 500-7000 ppm, with no added water, it originates from the gas from the combustion step, which has been washed and dried and used in part for oxychlorination, but also essentially for calcining; PA1 e) in the axial calcining zone, the catalyst is treated for 45 min to 80 min in a counter-current at between 350.degree. C. and 550.degree. C. at a pressure in the range 3 to 8 bars, using a portion of the gas originating from the washing circuit and a drying zone, the gas not containing more than 100 ppm of water.
A number of patents concern the regeneration of such existing catalysts, in particular United States patents U.S. Pat. No. 4,980,325 and U.S. Pat. No. 5,053,371. In those patents, the oxychlorination and combustion zones are separate so as to allow the catalyst to pass but not gas, and there is a circuit for recycling the gases from the oxychlorination step. U.S. Pat. No. 5,053,371 describes the operating conditions: 3-25% of oxygen in the gas introduced into the oxychlorination step, a chlorine content in the oxychlorination zone of the order of 500 ppm molar and a low water content which originates from the catalyst and gas from the calcining step. in U.S. Pat. No. 4,980,325, the oxygen originates solely from the oxygen-enriched gas which is introduced to the calcining step.
We have established that while they re-introduce chlorine into the catalyst, those operating conditions for the oxychlorination step, do not ensure correct re-dispersion of the bimetallic phase. This results in a degradation of the catalytic action over time.
Thus, a gas management which could precisely control the operating conditions of the oxychlorination step and preferably also those of the oxychlorination step was researched.
The process and unit of the invention satisfy these objectives.